The present invention relates generally to radio frequency (RF) transmitters and, more particularly, to RF transmitters with multiple power level capability.
Solid-state amplifiers, such as RF power amplifiers, are widely used for wireless communications. The goal of these power amplifiers is to achieve high operating efficiencies.
Various techniques have been employed to provide multiple power level capability for wireless communications. One technique is to place an attenuator before the power amplifier to reduce the output signal level from the amplifier. Because, however, the power amplifier is configured to operate efficiently only at a specific, typically, peak instantaneous power level, the amplifier""s operating efficiency is limited.
Envelope elimination and restoration (EER) power amplifiers have been known for many years. Generally, the efficiency of the EER amplifier is controlled by using circuit elements to modulate a supply voltage. A common EER amplifier topology uses a switching mode power supply to modulate the drain voltage of a single stage RF power amplifier. A pulse width modulator (PWM) feeds the power amplifier. The output of the amplifier is then passed to a low-pass filter, and the filtered output is used to modulate the drain of a MOSFET configured as a single-stage class D RF amplifier. The PWM applies a simple feedback mechanism to shape its output. A conventional EER amplifier only operates at very low bandwidth signals, such as 3-30 kHz, and does not operate efficiently over a wide range of average power output levels. This can contribute to low operating efficiencies. For low bandwidth signals, a conventional EER amplifier also fails to reduce the amount of in-band quantization (i.e., switching) noise in the output signal. The result is that the output signal must be sampled at very high rates, which causes significant losses in the output signal.
In general, the present invention is directed to a highly efficient RF transmitter including a power amplifier circuit with an extended efficient power control range. The preferred RF transmitter may be configured to operate using a wide variety of communication standards. An RF transmitter implementing the invention is highly efficient for nonconstant-envelope modulation formats and can operate over extended control ranges for high frequency/wide bandwidth operations, such as wireless communications.
In one aspect, the invention is directed to an RF transmitter that includes a signal processing block. A signal input to the signal processing block is up-sampled or interpolated. A supply modulator is coupled to an envelope output signal from the signal processing block, and a quadrature mixer is coupled to constant-envelope outputs from the signal processing block. A power amplifier circuit including a plurality of cascaded amplifiers is coupled to the supply modulator, and an attenuator is coupled between the modulator and the power amplifier circuit.
Implementations of the invention include one or more of the following. The RF transmitter may include switching circuitry configured to bypass one or more of the amplifiers, when the supply modulator operates in a low power mode. The amplifiers may be used to impart a gain to an output signal of the attenuator. The RF transmitter may also include a modulator connected between the envelope output and the supply modulator, where an output of the modulator further includes a high speed binary pulse stream.
In another aspect, the present invention is directed to an RF transmitter that includes a signal processing block responsive to an input signal. A supply modulator receives an envelope output signal from the signal processing block, and a quadrature mixer that receives a two component constant envelope signal from the signal processing block. An attenuator is configured to attenuate the output of the quadrature mixer, and a power amplifier circuit is connected between the attenuator and the supply modulator.
The preferred transmitter has many benefits. The preferred transmitter may be implemented in a wireless handset. This wireless handset may have longer talk times and smaller and lighter batteries. Further, the preferred transmitter reduces the size and cost of the handset and makes the handset more attractive to the wireless customer. The extended efficient power operating range of the transmitter also lengthens the handset""s battery life.